There are a number of processes for converting lignocellulosic biomass into liquid streams of various sugars. Certain preferred processes are based on supercritical water (SCW) or hot compressed water (HCW) technology, which offer several advantages including high throughputs, use of mixed feedstocks, separation of sugars, and avoidance of concentrated acids, microbial cultures, and enzymes. Processes using hot compressed water may have two distinct operations: pre-treatment and cellulose hydrolysis. The pre-treatment process hydrolyzes the hemicellulose component of the lignocellulosic biomass and cellulose hydrolysis (CH) process hydrolyzes the cellulose fibers. The resultant five carbon (C5) and six carbon (C6) sugar streams are recovered separately. The remaining solids, which consist mostly of lignin, are preferably recovered, such as through filtration, and may be used as a fuel to provide thermal energy to the process itself or for other processes. Lignin has the combustion heat of 26.6 KJ/g, and holds highest energy among all natural polymeric compounds that contain carbon, hydrogen and oxygen. In energy, lignin is equivalent to ethanol, which also contains carbon, hydrogen and oxygen, and has the combustion heat of 30 KJ/g. However, for a given volume, lignin's combustion heat is approximately 1.5 times as much as that of ethanol, because of lignin's higher density. http://www.altenergymag.com/emagazine/2009/06/lignin-as-alternative-renewable-fuel/1384). Thus, lignin serves as a useful renewable energy source.
Lignocellulosic biomass contains cellulose, hemicellulose, and lignin, along with minor amounts of proteins, lipids (fats, waxes, and oils) and minerals. About two thirds of the dry mass of cellulosic materials is present as cellulose and hemicellulose with lignin making up the bulk of the remaining dry mass. Lignin is a cross-linked racemic macromolecule with a molecular masse in excess of 10,000 Daltons. It is relatively hydrophobic and aromatic in nature. The degree of polymerization in nature is difficult to measure, since it is fragmented during extraction and the molecule consists of various types of substructures that appear to repeat in a haphazard manner. Different types of lignin have been described depending on the means of isolation. “Lignin and its Properties: Glossary of Lignin Nomenclature,” Dialogue/Newsletters Volume 9, Number 1, Lignin Institute, July 2001.
There are three monolignol monomers, methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. K. Freudenberg & A. C. Nash (eds) (1968). Constitution and Biosynthesis of Lignin. Berlin: Springer-Verlag. These lignols are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringal (S), respectively. W. Boerjan, J. Ralph, M. Baucher (June 2003). “Lignin bios”. Ann. Rev. Plant Biol. 54 (1): 519-549. Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of dicotyledonous angiosperms is more often than not a mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three. Id. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants. J. Ralph, et al. (2001). “Elucidation of new structures in lignins of CAD- and COMT-deficient plants by NMR.” Phytochem. 57 (6): 993-1003.
Impurities may be introduced into lignin via processing of the lignocellulosic biomass. Since lignin compositions may be used as a fuel in the SCW or HCW process or other processes, they preferably have a low level of contaminants or impurities that contribute to health, environmental, and safety concerns. For example, it is highly desirable to have no or only a low level of compounds containing sulfur in the lignin composition, as the presence of sulfur may contribute to SOx emissions, when the lignin is combusted. In other applications, low levels of sulfur may also be desirable if lignin is chemically converted through a catalytic process to a downstream product or a derivative. Low levels of sulfur within the final product may also be desirable from product acceptance criteria, or low levels of sulfur may help prevent premature catalyst deactivation for such chemical conversions.
Accordingly, the invention is directed to lignin compositions having low levels of impurities, as well as other important ends.